Method and apparatus for handling tubular members at offshore locations



April 28, 1970 H. CARGILE, JR 3,508,409

METHOD AND APPARATUS FOR HANDLING TUBULAR-MEMBERS AT OFFSHORE LOCATIONS1 Filed Dec. 26. 1967 2 Sheets-Sheet i M/m/A; mm: was (ONT/P04 BY iQMM Afro/m5 y 3,508,409 METHOD AND APPARATUS FOR HANDLING'TUBULAR MEMBERSApri 8. 1970 N. H. CARGILE, JR

AT OFFSHORE LOCATIONS 2 Sheets-Sheet 2 Filed D90. 26. 1967 /e J).lNVEN/UR.

BY g

Arrow/3V nited States Patent 3,508,409 METHOD AND- APPARATUS FORHANDLING TU- BULAR MEMBERS AT OFFSHORE LOCATIONS Neil H. Cargile, In,Cargile Road, Nashville, Tenn. 37205 Filed Dec. 26, 1967, Ser. No.693,597 Int. Cl. E02d 21/00; B63b 35/44; E21b 15/02 US. Cl. 61-46.5 6Claims ABSTRACT OF THE DISCLOSURE Method and apparatus for use with aworkover or drilling rig, preferably water-borne, by a floatablestructure with the rig normally cantilevered over the edge of the vesselabove a well head or cased opening, said apparatus incorporating a meansbetween the rig and the well for directing a generally upward forceagainst the rig wherein the magnitude of the force is related to thehook load acting on the rig, to cancel the dynamic loading on the rig.

Summary of problem and invention Offshore petroleum work is generallydivided into two classes of activities, the first of which is directedto the drilling of oil wells, and the second of which is the servicingof the well and completion. While the dividing line between the twoactivities is not hard and fast, for purposes of the present disclosure,workover and completion work are grouped together and generally definedas those activities involving a water-borne vessel or other fioatablestructure having lighter derricks and draw works which are oftencantilevered over the edge of the vessel. Drilling activities arenormally understood. In the area of activities presently proscribed,problems exist in the use of a workover or drilling rig as will benoted.

Describing the problem in the context of a workover rig, the rig isfloated to the site of the completed well or other well headinstallation, and the workover rig is positioned above the well head. Ifthe task at hand is one of pulling the tubing string, typically thetubing string is pulled by the draw Works and derrick apparatus in awell known manner. In view of the depth of wells being drilled, it willbe understood and appreciated that the hook load acting on the rig issubstantial. In further view of the fact that the heavy load actingdownwardly on the rig is displaced from the center of strength of thevessel or barge, often cantilevered to one side, it becomes readilyapparent that enhanced strength and support on the vessel itself isrequired. One previous solution is use of an equal and diametricallyopposite weight on the barge or vessel to prevent its tipping toward thewell head. This is ordinarily obtained by the use of ballast, such asrocks loaded aboard the vessel, or the use of floodable tanks which arefilled with a view of providing the counter-balance weight. Absent thecounterbalance, the jack-up leg or other strutural support beneath thevessel is loaded heavily adjacent the cantilevered workover rig, whereasthe remote legs are lightly loaded, if at all.

It will be appreciated that the disproportionate allocation of the loadto the jack-up legs is undesirable, and generally requires perhapstwenty-five percent to fifty percent more structural steel in thejack-up legs. Because the entire vessel is placed under torque, thelaterals or transverse members in the vessel itself must be reinforcedto prevent warping. Therefore, the devices to which the presentinvention relate have only solved the problem of off-center loading invessels by the brute force techniques of placing a disproportionatelyoverbuilt structure in the water, or the addition of grossly heavyballast which, while it counteracts the torque, creates a substantiallyinlice creased dead weight load acting on the underpinning or othersupports of the barge structure.

Since the present problem occurs without regard to the mode of operationof the vessel, the device of the present invention is applicable tooff-shore vessels of all designs, including drilling vessels, and forpurposes of illustration and not as a limitation, it will be describedhereinafter mounted on a jack-up barge. With a view of the foregoingproblems in mind, the present invention is summarized as providingimproved means and apparatus for use with a cantilevered vessel whereina means providing a generally upwardly directed force acting on thederrick offsets the hook load, and, in effect, returns the load on thederrick, draw works and other equipment to the well head or casing, itbeing noted that the Well head or installed casing is typicallyconstructed and arranged to support this weight at all other times.Therefore, one object of the device of the present invention is toprovide a new and improved off-shore vessel method and apparatus whichavoids cantilevered or off-center loading of the vessel when engaged inworkover, completion, or drilling. operations Another object of thepresent invention is to provide a new and improved apparatus permittingthe use of symmetrically built oif-shore barges which do not requirecounterbalancing ballast tanks or the like, and wherein the structurallegs or other support means are designed to merely support the barge andnot excessive weights or torques on the barge.

A related object of the present invention is to provide a new andimproved apparatus which shifts the dynamic loads resulting from use ofthe rig carried on the barge and returns or restores such loading to thewell head or well casing which normally carries the weight of the tubingstring, and is sufficiently strong for such use.

Another object of the present invention is to provide a new and improvedmethod of pulling a tubing or drill string from a water-located wellwhereby the dynamic load required for pulling the tubing or drill stringis isolated at the well and not in the apparatus or vessel used forpulling the tubular members.

Other objects and advantages of the present invention will become morereadily apparent from a consideration of the included drawings andspecification wherein:

FIG. 1 is a side view of the present invention installed on an off-shorevessel; and

FIG. 2 is a side view of the present invention installed beneath arotary table to support the weight of the drill string locked in thebowl by the slips.

In FIG. 1, attention is first directed to the off-shore vessel 10 whichis shown supported on a jack-up leg or spud extending to the sub-soil.The numeral 14 indicates the well protector which, in the currentexample, extends to a point above the water line 16. The casing orprotector 14 extends upwardly to a well head 18, the well head 18 beingsurrounded or even incorporated in a pile jacket 20 which includes anumber of structural members extending to the soil beneath the 'body ofwater. A workover means indicated generally by the numeral 24 is carriedby the barge or vessel 10, and is adapted to be positioned above thewell head 18 for pulling a tubing string 26 from the well. Of particularinterest to the present invention is the placement or location of forcecreating means indicated generally at 28, and extending between the wellhead 18 and the workover means 24 whereby a force is created actingupwardly on the workover means to reduce or even cancel the load of thetubing string 26 acting on the workover means 24.

As noted before, the workover means is shown in FIG. 1 and will be firstdescribed and thereafter, the invention in use with a drilling rig willbe noted.

Considering the invention more in detail, it should be noted that thebarge 10 s a conventional vessel known in the art, and is adapted to bemoved, towed, or otherwise transported on the body of water 16 to thelocation of the well 14. The precise nature of the barge is of noparticular significance to the present invention, and for this reason,many details are omitted. The illustrated embodiment is shown withjack-up legs 12 which elevate the barge above the surface of the water16 whereby the weight of the barge is borne on several jack-up legs 12,not fully shown in the first illustration.

Normally, the jack-up legs 12 are symmetrically located about the vessel10 to carry a proportionate load when the barge is elevated from thewater surface. However, such symmetrical loading is not possible whenthe workover means 24 is cantilevered over the edge of the barge 10without the utilization of the present invention. As noted hereinabove,the offset or cantilevered positioning of the workover rig places anon-symmetrical load on the barge legs which either creates bendingtorques in the long, slender legs, or which is only offset by the use ofsubstantial balance weights at the other side of the vessel, therebyincreasing the dead weight carried by the legs. Since neithercircumstance is desirable, the device of the present invention isdescribed hereinafter as providing a means whereby the hook load actingon the workover rig is cancelled by an approximately equal and oppositeforce applied to the rig, the force communicating the weight of thetubing string 26 back to the well head 18 which is routinely constructedand arranged to support this weight at all other times.

Note should be taken that the device of the present invention is notlimited to the well head apparatus shown in FIG. 1. The view illustratesa pile jacket platform extending above the surface of the water 16. Inactuality the well head 18 has many configurations located above orbelow the water line subject only to the requirement that a suitablestructure support the standard well head equipment and provide supportto the tubing hangers, both requirements being well known. In view ofthe variety of well head installations, the platform 20, itsunderpinnings, and the other structures are shown for purposes ofillustration and not limitation in the present disclosure.

For convenience to the vessel 10 during navigation, the workover means24 is preferably mounted on a pair of skid rails 32. The rails 32 aremovable on the deck of the vessel 10 to permit the workover means 24 tobe moved inboard of the vessel to improve its stability when the vesselis riding in the water. At a selected site, the rails 32 are moved withrespect to'the vessel locked in position by suitable means such asclamps, skid beams,

barge superstructure or the like for extension out over 4 the water. Therails 32 are generally of I-beam construction construction with crossbracing (not shown) located there-between wherein the rails 32 form aplatform clamped or secured with respect to the vessel and the platformis extended to carry the workover means over the water. The rails 32 areextended over the water to whatever length needed. This carries thedevice of the present invention to the illustrated position.

The workover means 24 includes a telescoping workover mast 36 which issupported on the platform and is movable therewith. The mast 36 ispreferably erected to the desired height when positioned over the wellhead 18. The mast carries conventional pipe handling apparatus includinga crown block 38, the traveling block 40, the appropriate swivel andhook; rotary drilling equipment, if needed, is included, and a rotarytable, the Kelly drive, and other apparatus are placed on the platform.The precise nature of the apparatus is subject to variation and is notedherein to describe equipment known and understood by those skilled inthe art. The workover or completion equipment is subject to variation.

A cable 44 is shown extending from a draw works 46 to the crown block 38and is utilized in the normal mafiner to raise or lower the travelingblock 40. The draw works 46 includes the conventional winch, motiveapparatus, and other needed equipment. The cable 44, at its dead end, istied to a hydraulic transducer 54, preferably a piston-cylinderarrangement secured to the platform and capable of forming a hydraulicpressure signal which measures the tension in the cable 44. Themeasurement of the tension in the cable 44 finds significant use in thepresent apparatus. It will be appreciated that the cable had isproportionately related to the hook load carried by the hook 42. Evenwhen the hook 42 is not engaged with the tubing, and there is sometension in the cable 44, it will be appreciated that the correspondingweight is relatively small in comparison with the massive loads placedon the hook by the tubing string, most especially when the tubing stringis struck in the well bore. As a matter of fact, hook loads as high asfive hundred thousand pounds sometimes occur.

The load transducer 54 is communicated by a hydraulic line 56 to anotherline 62 input to a number of hydraulic jacks 64. As the pressure in theline 56 is increased by the hydraulic transducer 54, the pressure inputto the jacks 64 is likewise increased. As suggested in the drawing, itis possible to use a hand control valve communicating with the mainhydraulic system aboard the vessel 10 (at a suitable high pressure)wherein the outlet pressure of the valve 60 is regulated to approximatethe pressure needed to offset the load on the derrick. Since the precisenature of the arrangement is subject to variation, the above notedarrangements are believed exemplary of apparatus suitable forinstallation in the present invention.

The several, preferably identical jack means 64 carried beneath theplatform are adapted to extend to the Well head 18. The jack means 64are secured at their upper end to the platform on which the workovermeans 24 is carried, and upon being fixedly attached thereto, access ispermitted for connection of the hydraulic cylinders 64 to the well head18. As a matter of convenience, it may be desirable to provide forpermanent installation of the hydraulic jacks 64 to the rails 32. As amatter of flexibility, it may be helpful to provide a universal adaptoron the piston rods extending from the jack means 64 whereby connectionwith the well head 18 is achieved without regard to the variations inshape or location of the well head 18.

It should be noted that the hydraulic jacks 64 need only be singleacting apparatus and capable of extension under pressure. Since weightis carried by the hydraulic jacks 64, the pistons are compressed intothe cylinders of the jacks by the quiescent load.

The drawing illustrates two jack means 64, although a greater number maybe used to maintain symmetrical loading on the well head 18. Because thedetails of the well head are dependent on many circumstances andfactors, it may be necessary to provide as many as four or more jacksfor transferring the load as will be described.

In many installations, the well head 18 is located beneath the surfaceof the water 16 while the platform of the barge 10 is sixty or more feetabove the surface of the water. In this circumstance, it is necessary tosecure suitable elongate members on the Well head (by welding orclamping) so that the extenders span the gap to the jacks '64. The jackmeans 64 do not need excessively long stroke because their primaryfunction is to apply force to the platform on which the workover devicerests, and not to move it.

While the foregoing describes the apparatus of the present invention, itis helpful to now describe the method of the present invention, assuminguse of the above structure although others means are available. Assumingthat the vessel 10 is in place, that the rails 32 are extended to carrythe workover means 24 to the vicinity above the well head 18 and thehydraulic means 64 are extended between the workover rig and the wellhead 18. Access is gained by suitable techniques to the casing 14 andthe tubing 26 is exposed. The pipe is hoisted upwardly by the hook meanin the well bore. The movement of the tubing 26 involves upward anddownward movement of the traveling block 40 in response to thedraw works46 which reels the cable 44 on the drum. It will be appreciated that thecolumn of tubing 26 is substantially heavy for a deep well, and, thelifting force is often increased should the tubing string be stuck inthe well bore.

After access is obtained to the upper end of the tubing string 26 andthe traveling block 40 is lowered sufficiently to engage the hook 42with the tubing string, the cable 44 hoists it upwardly and free of thecasing 14. At this juncture, theload on the hook is transferred to thevessel through the supporting structure of the rails 32; as notedbefore, this load reacts to the detriment of the barge 10 by eitherunbalancing the stable footing of the barge 10 by putting too muchweight on one leg, or alternatively, by requiring balancing ballastweights which increases the total dead weight of the vessel andtherefore increases the structural requirements of the jack-up legs 12.But, with the present invention, load on the vessel 10 at its edge orcorner as shown in the drawing is offset in the following manner.

As the cable 44 is pulled taut, indicative of an increased hood load,the load transducer means 54 senses the increase in tension andincreases the pressure in the hydraulic line '56. The hydraulic line 56communicates the control pressure to the hydraulic jacks 64. Since theincrease in pressure in the line 62 is proportional to the tension inthe line 44, the pressure in the hydraulic jacks 64 is likewiseproportional to the tension in the cable 44. This means that thehydraulic jacks push upwardly on the rails 32 with a force approximatelyequal and opposite to the forerequired to unstick and lift the tubingstring engaged by the hook means 42. The net result is that the forceson the platform that extends over the side of the barge 10 cancel oneanother. Moreover, the downward urging of the jack means 64 returns theweight of the tubing to the well head 18 and the well head 18, beingconstructed to carry the weight of the tubing string 26, amply supportsthe load transferred from the vessel 10. Noting the fact that the wellhead 18 is constructed to carry this weight at all times, the method ofthe present invention provides a force equivalent to the weight actingon the well head 18 to thereby maintain the normal static load of thewell head and to remove the dynamic load from the workover rig 24.

The above described method is preferably dynamic in operation as will benoted. For instance, should the tubing string 26 hang in the well bore,and should the tension in the cable 44 be increased unusually high topull the tubing string 26 free of the obstruction, the hook load issubstantially increased. The transducer 54 is sensitive to the increasein tension in the cable 44, and the pressure in the line 62 is increasedaccordingly to thereby increase the pressure within the jack means 64.Since the jack means are constrained against physical move ment, theincreased pressure therein increases the equal and opposite forcesacting at the upper and lower ends of the jack means whereby theplatform is reactively balanced against the cantilevered load, and thedynamic increase is communicated to the well head which carries theburden. Should the hook load drop suddenly, dynamic relief of thepressure build-up in the hydraulic system permits the platform carriedon the rails 32 to continue stabilized without regard to its loading.

While the foregoing has been describing the preferred apparatus andmethod of the present invention, several alternatives should be notedwith regard to both the device and method of the present invention. Forinstance, it may be more convenient to obtain a proportionate indicationof the load acting on the hook by placing a load cell at some point inthe derrick 36. For instance, strain gauges attached in the lower legsof the derrick 36 provide an electrical signal responsive to the stressacting in the derrick, a signal proportional to the load acting on thehook 42. Moreover, it might be convenient to sense the loading on theaxle of the crown block 38 to cite a second alternative load sensingpoint. While many techniques of load sensing are available and may beadopted by one skilled in the art, the nature of the proportional signalshould also be considered. It is acceptable to use av hydraulic signalas provided in the present system, or, if desired a proportionalelectrical signal from a strain gauge or the like. In the case of anelectrical signal, suitable control amplifier means are known in the artto provide a control signal to a pressure regulator, pump, controlledvalve, or the like, whereby the pressure in the conduit 62 is madeproportional to the load acting on the hook of the draw works. Likewise,it is not absolutely essential to use hydraulic jacks as shown in thepreferred embodiment. One suitable structure would be the implementationof mechanical jacks such as screw-type jacks, or other similar devices.However, it would appear that engineering economies are achieved byhydraulic jacks.

Attention is next directed to FIG. 2 of the drawings which illustrates aplatform adapted to be cantilevered over the side of a vessel (not shownas a matter of clarity). Assuming the use of the vessel such as shown inFIG. 1, the platform 100 is preferably adapted for use with a drillingrig indicated in dotted line at 101. The drilling rig preferably hasfour load bearing upright supports which are each received on andsupported by a load cell 102 positioned beneath each of the cornermembers. The load cells are deployed to support the entirety of the loadacting on the drilling rig 101. As will be understood, the drilling rig101 reactively returns the load to the load cells 102.

The view of FIG. 2 omits the rotary table, supporting deck work for therotary table, and the bowl for receiving the slips for locking the drillstring in position. These details are omitted to show the hydraulicsystem in accordance with the teachings of the present invention.However, FIG. 2 does include an I-beam 104 on which is received deckplating 105 which extends to the center of the platform 100 forsupporting the rotary table.

It will be appreciated that the weight of the drill string is supportedby the drilling rig 101 during drilling operations. However, during theinterval which the drill string is supported by slips wedged into thebowl, the weight of the drill string is transferred from the drillingrig 101 to the rotary table, and thence to the I-beam supports 104. Itwill be understood that suitable beams are placed beneath the deckplating 105 to transfer the weight on the rotary table, including thedrill string when at rest, to the load cells 102 which are deployedunder the four corners of the drilling platform 105. The I-beam supportspreferably extend to the four corners and cross-bracing is provided asneeded. By the use of the described structure, it will be understoodthat the platform 105, which includes the rotary table, is, in effect,rested on the four load cells shown at the corners.

One factor which alters the load on the cantilevered platform 100 is thedeployment of drill pipe or other tubular members resting in the derrickwith the greater portion of their weight acting as the lower end inconventional pipe racks as will be understood by one skilled in the art.This is illustrated somewhat schematically in FIG. 2 wherein a pluralityof pipes is indicated at 108, the intention being one of resting thedynamic load of the pipe on the present invention whereby the load istransferred from the platform 100 proper to the protector casing as willbe noted.

Attention is next directed to the hydraulic apparatus shown in FIG. 2. Asource of fluid under pressure is provided by means, not shown, throughappropriate accumulators, valves, and the like to a hydraulic line 110.The hydraulic line communicates by way of a line 111 to a dead-end loadsensor 112. As was shown in FIG. 1, one technique of obtaining the loadacting on 7 the derrick 101 is to sense the tension in the dead end ofthe line indicated at 113. The cable 113 extends upwardly into thederrick and is connected through the crown block and traveling block tosupport the entirety of the weight of the tubing string. As the hookload increases, the tension in the cable increases and this is reflectedat the dead end of the cable 113 wherein the increased tension pulls onthe hydraulic .transducer shown at 112. The transducer means 112converts the tension in the cable 113 to hydraulic pressure ascommunicated through the line 111. By way of example and not limitation,a single acting hydrauli cylinder filled with pressure fluid increasespressure in the line 111 in response to increased tension in the cable113. The increase in pressure is communicated to the line for responseas will be described momentarily.

A second branch line 115 has a manifold input to the lurality of loadcells 102. The load cells 102 are connected in parallel with one anotherand to the line 115. Again, the load cells can be aptly described assingle.- acting piston and cylinder arrangements wherein the weight orload acting. thereon increases pressure in the line 115.

The branch lines 111 and 115 supply control pressure through the line110 and through the valve 16 to each of a number of downwardly extendinghydraulic cylinder means 120. While FIG. 2 illustrates two hydraulicjacks indicated at 120, it will be appreciated that a symmetricalarrangement incorporating two or more hydraulic jack means 120 isdesirable. For sake of clarity of the drawings, additional hydraulicjack means 120 have been omitted from FIG. 2; however, the disclosurerelating to the illustrated hydraulic jack means is believed adequate.

The pressure in the line 110 causes the jack means 120 to extend in themanner described with respect to FIG. 1. It will be appreciated that theupper end of the jack means 120 is connected at 121 by suitable mountingbrackets to the cantilevered platform 100. The lower end of the jackmeans 120 is connected with a load ring 122 which is secured about andrelative to a protector casing 124. As shown in FIG. 1, it will beunderstood that the protector casing extends part-way into the water andsoft bottom, and is sufliciently stout and stable to support the loadplaced thereon. Protector casings are customarily installed at mostdrilling installations to prevent pollution of fresh water formationsand other undesired communications up or down the well bore. As wasshown in FIG. 1, a platform may or may not be present surrounding theprotector casing 124, and it will be further noted that a protectorcasing 124 and the load ring 122 are located below or above the surfaceof the water depending on factors not related to the present disclosure.

In operation, the structure shown in FIG. 2 functions somewhat in themanner as the structure shown in FIG. 1, the primary difference beingthat the derrick or rig shown in FIG. 1 is normally used for workover orcompletion operations, while the structure shown in FIG. 2 is primarilyused for drilling wells. Of course, the two together indicate the use ofthe present invention with well bore activities of any description ornature, as will be understood. More specifically in FIG. 2, a load issensed either by the dead-end transducer 112 or by the load cells 102 tocommunicate an increase in pressure to the conduit 110. The increase inpressure is communicated to the jack means 120 which are positionedbelow the platform, and which are connected to a fixed memher, the fixedmember being the load ring 122 which is carried on the protector casing124 and is fixed in position and adapted to support the load. As thepressure increases, the jacks 120 are extended. As the jacks extend,they apply a force to the lower side of the cantilevered platform 110which is approximately equal and which has a vector directionapproximately opposite to that of the dynamic load acting on theplatform 100. It is immaterial that the load acting on the platform 100is derived from the'hook load in the derrick 101 astransferred by thecable 113 to the dead end transducer 112 or through the legs of thederrick at 101. In either event, a load is transferred to appropriatetransducers which increase the pressure in the line in relationship tothe dynamic load. Moreover, the load of a full rack of pipe carried inthe derrick is likewise detected and the hydraulic jack means arelikewise extended in response to such a load. With the foregoing inview, and somewhat similar to the operation of the structure shown inFIG. 1, it will be understood that the drilling rig operates similar tothat shown in FIG. 1.

With the foregoing in view, it will be appreciated that the scope of thepresent disclosure applies to any suitable means for deriving the forceacting on the apparatus, using the signal indication thereof to therebycontrol the force applied generally upwardly and oppositely beneath therig, and also co-acting on the well head 18.

While numerous changes or variations in the present invention aredescribed above, and since others are known in the art, it is desirableto describe the invention broadly, limited only by the scope of theclaims appended hereto.

What is claimed is:

.1. A method of dynamically loading a workover, completion or drillingrig which can be used when extended toward or over the side of a bargeor vessel for carrying the rig; comprising the steps of:

(a) sufficiently aligning the rig with respect to a well thead;

(b) loading the rig by using same in a conventional manner;

(c) sensing the dynamic loading acting on the rig;

and

(d) applying a force proportionally related to the sensed dynamicloading and against the rig in a direction tending to cancel the dynamicloading thereon.

2. The method of claim .1 further including the step of couplinghydraulic force-applying means between the rig and well head before therig is loaded whereby said proportionally related force is developed byapplying a hydraulic pressure to the force-applying means that isproportionally related to the dynamic loading on the rig.

3. The method of claim -1 further including the steps of couplinghydraulic force-applying means between the rig and well head before therig is loaded; sensing the tension in the cables of the rig; anddeveloping a hydraulic pressure on the force-applying means that isproportionally related to the cable tension for applying saidproportionally-related force between the rig and wellhead.

4. For use in offshore operations with a vessel carrying a rig, theimprovement comprising:

(a) a platform adapted to be positioned over the edge of the vessel, therig being mounted thereon;

(b) means for forming a force acting on the rig tending to counteractthe load acting on the rig and including hydraulic jack means extendingdownwardly from said platform; and

(c) adaptor means for connection to said jack means and constructed andarranged to place a load relative to the Well head acting downwardlysuch that support is provided to said platform by said jack means.

5. The invention of claim 4, further including:

(a) transducer means for sensing the tension in the cables of the rigand for forming a signal proportional to the load thereon; and

, (b) means connected to said transducer means and to said hydraulicjack means for forming a hydraulic pressure therein for effectivelyreturning the weight of the load acting on the rig to the well head.

6. The invention of claim 4, further including:

(a) load sensing means adapted to be positioned be- .neath the loadacting on the derrick of a rig;

9 10 (b) means connected to said load sensing means and 3,201,945 8/1965Sutton 61-465 to said hydraulic jack means for controlling a hy-3,368,618 2/1968 Knox 166-.5 draulic pressure therein for effectivelyreturning the 3,421,581 1/1969 Geijn 17527 X weight of the load actingon the rig to the well head.

5 STEPHEN J. NOVOSAD, Primary Examiner v References Cited UNITED STATESPATENTS US. Cl. X.R.

2,334,992 11/1943 Crake 61-465 166-.5; 175-5, 24 3,189,098 6/1965 Haeber166-.6

